This invention relates to the field of contoured molded pulp products, such as containers for food or the like, which have a plastic liner on at least one surface, usually the upper food containing surface, and provides for the first time, it is believed, such a product which can be subjected without damage to temperatures of up to 400.degree. F. when empty for a short time, and for as long as 45 minutes when the contained food or other substance is at room temperature of below at the outset.
Prior to the present invention, it has long been known that various thermoplastic materials, including polyesters, can be bonded by heat and pressure to a pre-formed and contoured body molded of fibrous pulp material. This provides an attractive and highly moisture resistant container, suitable for purposes such as plates, bowls and the like for serving food. These products as previously known are not suitable, however, for use as ovenable containers involving exposure to high temperature for long times because, among other deficiencies, the plastic liner either melts or shrink-separates away from the molded pulp base when exposed to high temperatures, and the pulp exhibits significant undesirable browning or even outright charring when exposed to such temperatures for long times. Representative examples of prior art patents disclosing such containers are as follows.
Stevens U.S. Pat. No. 2,590,221 (Mar. 1952) discloses a method of fusing or bonding a sheet of thermoplastic material, such as polyethylene, to a contoured molded pulp body by heating the face of the thermoplastic material to be bonded with the pulp to the fusion point while allegedly maintaining the opposite face at a temperature below the fusion point.
Amberg U.S. Pat. No. 3,616,197 (Oct. 1971) discloses a method of adhering a liner of polypropylene film to a contoured molded pulp plate by means of an intermediate adhesive layer, such as a pigmented polyamide ink or a pigmented resin emulsion.
Singer U.S. Pat. No. 3,657,044 (Apr. 1972) discloses a method of bonding a sheet of thermoplastic material, including polyester, to a contoured molded pulp body by the use of superatmospheric pressure to force the thermoplastic against the molded pulp.
Amberg U.S. Pat. No. 3,697,369 (Oct. 1972) discloses a method of fusing or bonding a liner of polypropelene film to a contoured molded pulp plate by means of an intermediate layer of polyethylene which secures the bond with the molded pulp.
South African Pat. No. 73/2988 (May 1972) discloses a method of bonding a previously coextruded laminate of polypropylene and polyethylene to a contoured molded pulp body.
More recently it has been discovered that ovenable containers suitable for use in the re-heating and/or further cooking of food in microwave and conventional ovens can be made by either press-forming them, or folding and adhering them, from a sheet of flat paperboard having a coating of substantially amorphous polyethylene terephthalate bonded by its own substance to at least one surface thereof. These food containers present several problems, however, such as the fact that the press-forming or the folding and adhering of the container from the flat sheet is expensive, and there is a danger that the coating will rupture at the bend or fold lines. Such containers usually are neither as strong nor as attractive as molded containers. Also, the press-formed or folded-up container necessarily has pleats or folds and/or corner gaps which are completely unacceptable in certain containers, such as muffin baking trays, and they preclude hermetic sealing of a plastic overwrap. Representative examples of prior art patents disclosing such containers are as follows.
Kane U.S. Pat. No. 3,924,013 (Dec. 1975) discloses the formation of an ovenable food container from a laminate which includes nonbrowning paperboard with a coating of substantially amorphous polyethylene terephthalate extruded as a hot resin directly thereon and bonded thereto by its own substance.
Middleton U.S. Pat. No. 4,147,836 (Apr. 1979) discloses the formation of an ovenable food container from a laminate which includes paperboard coated, by a process which includes subjecting the paperboard to a corona discharge, with polyethylene terephthalate, noting that the crystallinity of the latter does not substantially affect the adhesion of the coating to the paperboard.
Thus, the problem heretofore unresolved by the prior art is the ability to provide an inexpensive, disposable container which is three-dimensionally contoured without pleats or folds, and which is capable, among other things, of holding food during exposure to high temperatures for long times in either microwave or conventional ovens without having the plastic liner melt or shrink-separate away from the contoured base, without any significant undesirable browning of the base, and without any other detrimental effect to the container or the food.